Retail and other establishments that store and sell refrigerated items frequently must be concerned with condensation problems. It is a common practice in commercial refrigerators and freezers, referred to below as coolers, to utilize a glass display door with a large window in it to provide easy access for a customer while allowing the customer to easily see what is inside the cooler. Frequently, the window makes up the majority of the door panel. Under adverse environmental conditions, condensation on the door frames and door window panes can be a problem.
For example, a door to a cooler in a store may be opened frequently by customers. When this happens, the inside of the door, which may be at a temperature of 2.degree. C. to 5.degree. C. (35.degree. F. to 40.degree. F.), is immediately exposed to the ambient air in the store. Depending on the temperature and humidity levels of the ambient air, condensation may form on the cold inside surfaces of the door. If the humidity is relatively high, heavy condensation may form almost immediately, which can completely obscure the view through the door glass. This obviously is detrimental to the purpose of the window, which is to provide a clear view inside the cooler. Additionally, the condensation may be heavy enough to cause the door to drip when opened. This is a particular problem in retail stores where it can create a hazardous puddle or get a customer wet and thus dampen their enthusiasm for that store.
In an effort to reduce or eliminate these problems, it has become a common practice to employ heaters in door windows and door frames of refrigeration equipment. These devices, which will be referred to as door heaters below, use small electrical heating elements to raise the temperature of the door glass or frame sufficiently that condensation is reduced or eliminated. Door heaters are used in both refrigerators and freezers, and both type of units will be understood to be included in the term cooler as it is used below. There is a significant energy cost associated with using such devices, however. It takes energy to power the door heaters, and the heat generated by these heaters must be removed from the refrigerated volume by the refrigeration system. The costs involved with door heaters can be substantial.